Conservation of Energy. 19 



If this engine makes 200 strokes per minute, then the work 

 it does per minute will be 



200 X 8,000 = 1,600,000 foot-pounds. 



21. Energy. Energy is the ability of a moving body to 

 do work and the amount of energy the moving body has 

 is measured by the amount of work it can be made to do 

 in coming to rest. If a weight suspended from a string 

 be drawn to one side and then released it will begin fall- 

 ing and acquiring velocity, and on reaching the lowest 

 level it will possess the ability of doing a certain amount 

 of work. That amount will "be enough to raise its own 

 weight through the height from which it fell in the same 

 time. If a bow is bent and the string is released against 

 the arrow it will recover its form of rest but in doing so 

 will impart to the arrow an amount of motion equal to that 

 which the bow acquired in straightening out. When 

 work is done in winding the clock the distorted spring 

 has the power to develop an amount of energy equal to 

 that expended in winding it up. In chopping wood the 

 action of the woodsman's muscles increases the amount 

 of motion in the ax until it falls upon the wood, when the 

 energy which has been imparted to it does the work of cut- 

 ting. 



We cannot exert pressure enough with the hand alone 

 to force the nail into the board, but by giving the muscles 

 an opportunity to act gradually upon the hammer it is 

 a simple matter to store in it enough energy to easily drive 

 the nail into the wood. When coal or wood is burned in 

 the fire-box of the engine and the heat developed converts 

 water into steam under high pressure in the boiler we have 

 still another case where energy is developed and accumu- 

 lated in the rapidly moving molecules of steam which drive 

 the piston whenever the valves are opened leading to it. 



22. Conservation of Energy. No discovery of modern 

 science is more fundamental than the fact that neither mat- 

 ter nor energy can be destroyed or created. One form 



